1. Field of the Invention
The present invention relates to the use of an immunogenic composition comprising a porcine circovirus type 2 (PCV2) antigen for the prevention and treatment of sub-clinical (chronic) PCV2 infections in animals, preferably in pigs.
2. Description of the Prior Art
Porcine circovirus type 2 (PCV2) is a small (17-22 nm in diameter), icosahedral, non-enveloped DNA virus, which contains a single-stranded circular genome. PCV2 shares approximately 80% sequence identity with porcine circovirus type 1 (PCV1). However, in contrast with PCV1, which is generally non-virulent, infection of swine with PCV2 has recently been associated with a number of disease syndromes which have been collectively named Porcine Circovirus Diseases (PCVD) (also known as Porcine Circovirus associated Diseases (PCVAD)) (Allan et al, 2006, IPVS Congress). Postweaning Multisystemic Wasting Syndrome (PMWS) is generally regarded to be the major clinical manifestation of PCVD (Harding et al., 1997, Swine Health Prod; 5: 201-203; Kennedy et al., 2000, J Comp Pathol; 122: 9-24). Other potentially related conditions reported in the literature include porcine respiratory disease complex (PRDC), porcine dermatopathy and nephropathy syndrome (PDNS), reproductive failure, granulomatous enteritis, and potentially, congenital tremors (CT-A11) and perinatal myocarditis (Chae, Veterinary J., 2005; 169: 326-336).
PCVD affects pigs between 5-22 weeks of age. PCVD is clinically characterized by wasting, paleness of the skin, unthriftiness, respiratory distress, diarrhea, icterus, and jaundice. In some affected swine, a combination of all symptoms will be apparent while other affected swine will only have one or two of these symptoms (Muirhead, 2002, Vet. Rec.; 150: 456). The mortality rate for swine infected with PCV2 can approach 50%. During necropsy, microscopic and macroscopic lesions also appear on multiple tissues and organs, with lymphoid organs being the most common site for lesions (Allan and Ellis, 2000; J. Vet. Diagn. Invest., 12: 3-14). A strong correlation has been observed between the amount of PCV2 nucleic acid or antigen and the severity of microscopic lymphoid lesions (Brunborg, 2004). In addition, correlation has also been found for the amount of nucleic acid or antigen in blood and the severity of the clinical symptoms (Brunborg, 2004; Liu, 2000; Olvera, 2004). Pigs suffering from PCVD have been shown to have viral loads that are higher than 106 genomic equivalents per ml.
In contrast to clinically apparent disease manifestations of PCV2 infection, sub-clinical PCV2 infections are thought to be present in those animals that are infected with PCV2 but are clinically asymptomatic. In general, a releationship exists between these forms of PCV2 infection since sub-clinical infections may easily transition into PCVD, and since convalescent animals may stay persistently (chronically) infected (see FIG. 1).
Recent observations have demonstrated that sub-clinical PCV2 infections are frequent events. The existence of sub-clinical infections has been demonstrated by both experimental and field studies. In laboratory studies it could be shown that PCV2 infection in individual pigs is not always associated with clinical signs or lesions (Harms et al., 2001, Vet. Pathol., 38:528-539). In addition, several field studies have shown that the incidence of PCV2 infected, seropositive herds is higher than the incidence of herds affected with PCVD (Olvera et al., 2004, J. Virol. Methods, 117: 75-80). Often, herds that have experienced an acute outbreak of PCVD remain PCV2 infected without showing any apparent clinical signs. According to the literature this form of sub-clinical (persistent) infection within a herd is also called “chronic” infection (Burch D., 2006, Pig International).
The economical impact of PCV2 in sub-clinically infected herds, if any, is unknown and has never been described so far. In particular, it was not known and no indication was ever given whether sub-clinical cases of PCV2 infections have any impact on growth performance of animals or, in general, on the overall health of the affected animals.
Approaches to treat PCV2 infections based on a DNA vaccine are described in U.S. Pat. No. 6,703,023. In WO 03/049703 production of a live chimeric vaccine is described, comprising a PCV1 backbone in which an immunogenic gene of a pathogenic PCV2 strain replaces a gene of the PCV-1 backbone. WO99/18214 has provided several PCV2 strains and procedures for the preparation of a killed PVC2 vaccine. However, no efficacy data have been reported. An effective ORF-2 based subunit vaccine has been reported in WO06/072065. Any of such vaccines are intended to be used for the vaccination/treatment of swine or pigs older than 3 weeks of age. None of these vaccines have ever been described for the prophylaxis or treatment of animals sub-clinically infected with PCV2. Moreover, such vaccines have not been described to confer immunity against PCV2 infection in sub-clinically infected groups of animals and/or to improve their growth performance.